Support getting a page preamble

Looks like the markdown generated starts out with a `## [NAME]` for everything thing documented, but there doesn't seem to be a way to get any preamble on to the page, especially a `# [TITLE]` since that likely will be used by some system.

The easiest might be to just support a `title` on the rule to insert this vs. and then let something like bazelbuild/skydoc#159 end up providing any content after the title before the listing of symbols.

The issue names no files or tests; trace the rule configuration and markdown page-generation entry point. Confirm how generated pages are assembled, then define done as supporting a page title and preamble before the documented symbol listing.

Written by the indexing model from the issue text.